Correction: TVA's initial estimate for Monday's pond spill has been changed to 3.1 million cubic feet of fly ash and water. The earlier figure given was 2.6 million cubic yards.

The ash that burst out of a TVA retention pond on Monday is the sort of waste that has gotten scrutiny from federal regulators and lawmakers in recent months.

The mixture in question was made of water and fly ash, a fine particle that's one of the by-products of burning coal to generate electricity. The ash is collected by scrubbers that aim to clean up emissions from power-plant smokestacks, and it includes trace elements of materials like arsenic and lead.

In June, a subcommittee in the U.S. House of Representatives held an oversight hearing to examine how the government should address the health and environmental risks of coal combustion waste, including fly ash.

Mary Fox, an assistant professor in the Department of Health Policy and Management in the Johns Hopkins Bloomberg School of Public Health, testified at this summer's hearing and said in written testimony that "Risks to human health are increased if people are exposed to coal combustion waste."

In an interview on Monday, Fox said she was reassured about reports that the Kingston spill was a release of wet ash because that means it wouldn't have gotten into the air as dust.

Fox said generally the concern about fly ash is in regard to long-term exposures - from leaching into the groundwater, for example - and said that when it comes to a spill, the main exposure issues initially will be for people who are cleaning up the mess.

"If it did impact someone's home," she said, "you'd want them to stay away from it and not try to be scooping it up with your own broom and dustpan, that kind of thing. It's not something that you'd want to try and clean up yourself."

In addition to state and local authorities, the Environmental Protection Agency also dispatched a coordinator and a contractor to the site. EPA is currently reviewing its regulation of coal combustion waste, including fly ash, and one of the documents involved in that process is a 2007 report which found 24 proven cases of damage to ground or surface water from the disposal of coal combustion wastes.

Lisa Evans, an attorney with Earthjustice, a California-based nonprofit law firm, said that in recent years, ash impoundments also have failed in Pennsylvania and Georgia. "Unfortunately this isn't an isolated instance," she said "The surface impoundments are not safe places to keep coal ash in general."

On the other hand, fly ash can be recycled into something useful. David Goss, executive director of the American Coal Ash Association, said ash that meets certain quality standards can be used as a substitute for Portland cement in concrete, and also has uses including soil and waste stabilization. Goss indicated that heavy metals are usually found only in low concentrations, and that they are diluted when stored in liquid.

While he wasn't familiar with the exact details of the Kingston spill, Goss said that typically the "levels of these concentrations are low enough not to deem them a public hazard."